Removal
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Remove master cylinder from the booster, do not disconnect the lines, and just move it out of the way .
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On the transducer, remove two speedometer cables.
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Remove three hoses from the transducer.
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Remove the two or three wire connector from the transducer.
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Locate the two small mount bolts located left and right of the bracket, remove them you will need a wrench on both sides cause of the lock nut!
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To get this off you will need some of these tools, if it’s original and a 80 Vette.
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On the bench, this is what the cruise control looks like.
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Mark the two casing halves for later reference and remove two small screws.
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The screws are in the slotted part of the casing.  The upper connector is for the output cable to the speedometer, and the lower one is from the transmission.
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Carefully split the casing, nothing should fall out.
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Carefully split the casing, nothing should fall out.
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Remove the small screw that holds the spring.
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 Careful the parts are fragile; you can see all the dirt that fell out.
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Now tap out the brass looking piece where the Speedo cable goes in, be gentle.
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Clean all the parts being careful, I used Brake Kleen, and a small burst of air to dry it, before re-assembly coat the gears and shafts with grease, and re-assemble.

Reassembly
Reassembly is a reverse procedure of the above.
Once the unit is reassembled, spin the end and see how free it is.  It should move easily.
Reinstall the unit and try it out !
Small Facts
A speed of 30 mph is required to turn the cruise control on.
It gets it's power from a 20 amp fuse in the fuse box under "gauges".
The vacuum source comes from the intake manifold, between the distributor and the carburetor.
Cruise Control Operation
Ron would like to pass on his sincere thanks to the Norwegian C3 Site for their co-operation and assistance and for permission to use the material below to assist others in understanding how the cruise control system in your Vette works.

Cruise Control
A magnetic coupling driven by the speedometer wire moves the clutch.
If speed is increasing clutch will move counterclockwise, if speed is decreasing it will move clockwise.
When cruise control is engaged (CC switch partially pressed), the relay activates and push the stem upwards, thus releasing the spring which grips tight around the clutch.
Also, the stem is operating a pneumatic sliding air switch which will apply vacuum to the actuator when cruise is engaged. (Moved upwards by the solenoid stem and connects incoming and outgoing vacuum lines.)
There is an electrical switch operated by the tab on the clutch which opens at speed below 35mph, thus preventing the relay to be activated.
This minimum lock-in speed can be adjusted by turning the transducer cover.
After the relay is activated, a small current through a 40ohm resistor keeps it in that position. (It takes much more power to activate a relay that to keep it activated).
To manually de-activate the CC you press the button all the way down. This release the relay by cutting it's power.
If you then release the button rapidly the relay won't have time to activate again (it's a fairly slow mechanical process)
If you release it slowly the relay will activate. Click HERE for a drawing of the complete system.
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Image Shows Transducer Interior
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The animation below shows how the transducer works.

The control valve (green) expose or covers the small square white holes that will let air enter the vacuum side through a tube connected to dark-red pipe. This tube is connected to the actuator.

(In reality, this cause a significant vacuum leak, not really a good solution chosen by the GM engineers....)

The transducer interior is kept at atmospheric pressure through a filter element at the bottom of the housing.

Speed to fast:

If speed goes up the clutch rotates counter-clockwise.

This will move the valve so that a larger area of the venting holes are exposed.

Since vacuum is applied to the actuator system through a small fixed orifice, the vacuum in the actuator will decrease and throttle will be reduced.

Speed to slow:

If the speed goes down the clutch rotates clockwise.

This will move the valve so that less area of the venting holes are exposed.

The vacuum in the actuator will increase and throttle will be opened more.

cruise_anim

Limitations of the system

The cruise controller is a simple proportional type with no error integration.

For a certain offset in speed there will a fixed movement of the throttle.

This will seldom be correct since you have to adjust the throttle response also relative to engine loads. (Just think of it: When you control the gas pedal yourself you adjust it not only in accordance with deviation from preferred speed, but you also consider engine load, humans are very good controllers...:-)

There is no feedback of throttle position.

This limitations is tried to compensate for by an adjustment of the position of the orifice tube (red).

It is threaded into the transducer housing and can be adjusted laterally by turning it.

This will adjust the throttle position when the CC is engaged. In essence you adjust the vacuum to the actuator when the clutch is in the center position.

In other words, the throttle will always be set to the same position when cruise is engaged.

The result of this is that you will only cruise at the lock-in speed at a certain speed and engine load.

Modern CC's have error integration which cancels out consistent offsets.

If another throttle position is needed to keep the speed you've got when engaging, the CC will have to adjust it.


 

Example:

Let say that the orifice tube is adjusted to a give a throttle opening that results in a cruising speed of 55mph at a flat road when clutch in center position.

If the cruise is engaged at 70mph a 10% higher throttle setting will be needed to keep that speed.

This can only be achieved if the car speed is reduced, causing the clutch to move 5degrees clockwise, thus delivering more vacuum to the actuator.

To achieve this rotation of the clutch the car speed must be 3mph lower than the lock-in speed.

Which means that the cruising speed will be 3mph below lock-in.

Visa versa in you lock-in at a speed lower than set point, the cruise speed will be above lock-in speed.

Same will also happen if engine load changes (down or uphill).

The actuator is pulling against the throttle return springs). In other words, if the total throttle spring force is not correct it will pull to much/to little. This can be compensated for by adjusting the orifice tube (red).

There is no vacuum regulator. Since manifold vacuum will vary, so will the throttle position. When the engine load is high (going uphill), vacuum will be low. Which , according to the example above, means you will cruise below the setpoint. 

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